Type mismatch (expected: 'Double', found: 'Unknown Class') with wildcard types

[abstratt]

    private query totalExpenses(toSum : Expense[*]) : Double;
    begin
        return toSum.reduce((e : Expense, sum : Double) : Double {
            sum + e.amount
        }, 0);
    end;

Error: Type mismatch (expected: 'Double', found: 'Unknown Class')

The problem here is that the parameter 0 is of type Integer, and the definition of reduce requires the second parameter of the closure and the initial value to have the exact same type, and they are double in the closure passed in. Since there is no single type we can unify the call values to, the result type (which is also the same wildcard type) is left unresolved (wildcard types don't have names internally, and are only referred to by context-specific aliases).

A few things here:

• the message is not really helpful, we could suggest the names of types we could take
• we are able to implicitly cast integers to doubles elsewhere, the exact match required here is counter-intuitive
• it seems the operation lookup works by falling back to the original operation definition (with wildcard types) - that should be an error condition